Abstract

Purpose: The immune checkpoint of the programmed death 1/programmed death ligand 1 (PD-1/PD-L1) pathway is believed to play an important role in evasion of host antitumor immune surveillance in various malignancies; however, little is known about its role in thymic carcinoma. This study investigated PD-1/PD-L1 expression and its association with clinicopathological features, the expression of immune-related proteins in tumor infiltrating lymphocytes (TILs), and patient prognosis.
Experimental Design: PD-L1 and PD-1 expression was evaluated by immunohistochemistry in 25 thymic carcinoma tissue specimens. Copy number alterations of the PD-L1 gene in 11 cases were assessed in formalin-fixed, paraffin-embedded material using quantitative real-time PCR.
Results: Compared to normal subjects, 3 thymic carcinoma patients showed an increase in PD-L1 copy number, whereas 8 did not. PD-L1 was significantly overexpressed in cases with copy number gain as compared to normal cases. High PD-L1 expression was associated with higher disease-free and overall survival rates as compared to cases with low expression. Prognostic analysis revealed low PD-L1 expression and high number of PD-1+ TILs as significant predictors of poor survival, together with Masaoka-Koga stage IVa/IVb disease and incomplete resection. In the quantitative analysis of TILs, PD-L1 expression correlated proportionally with the number of infiltrating cytotoxic T lymphocytes.
Conclusions: Here, for the first time, we report that PD-L1 and PD-1 expression might be useful prognostic predictors in thymic carcinoma. Further studies are expected to substantiate the prognostic value of PD-L1 and PD-1 expression, and the potential efficacy of targeting the PD-1/PD-L1 pathway in thymic carcinoma via immunotherapy.